There are several methods available for testing the operation of fuel injectors in internal combustion engines. Mechanics often use stethoscopes to listen to the sounds made by fuel injectors. A clicking sound emitted by an injector indicates that the injector pintle is moving. This method will detect injectors that stopped responding altogether, but will miss partially failed injectors. Also, this method cannot be used on injectors that are not accessible by the stethoscope because they are hidden under the intake manifold or under other engine components.
U.S. Pat. No. 6,668,633 discloses a battery-operated fuel injector tester with a probe attached to a pistol-shaped handle. When the probe of the tester is in contact with a tested injector on an idling engine, a light emitting diode flashes and an audible sound is emitted each time the pintle within the fuel injector opens. This tester will detect injectors that stopped responding altogether, but will miss partially failed injectors. Also, this method cannot be used on injectors that are not accessible by the probe because they are hidden under the intake manifold or under other engine components.
U.S. Pat. No. 4,523,458 discloses a fuel injector tester for injectors used in diesel engines. It uses a transducer comprising a piezoelectric crystal sandwiched between two magnets. The transducer is attached magnetically to a tested injector and displays on a bar graph the intensity of the mechanical impulses it measures. This method cannot separate the injector opening transient from the injector closing transient, it does not provide any information on the length of time when the injector valve was open, and it cannot be used on injectors that are not accessible by the transducer because they are hidden under the intake manifold or under other engine components.
U.S. Patent Publication Application No. 2006/0101904 discloses a system where a fuel pressure sensor is installed on the fuel rail and senses fuel pressure fluctuations associated with the operation of the fuel injectors. This method will detect a fuel injector that has failed altogether because the fluctuation expected when that injector was scheduled to open and inject fuel will be missing. However, this method is not accurate enough to reliably detect partially failed fuel injectors.
U.S. Pat. No. 5,747,684 discloses a method for determining the opening and closing times for automotive fuel injectors for use by the engine electronic control unit (ECU) to more accurately control an injector stroke, thereby improving engine performance. This method is based on analyzing the energy content of the acceleration of the injector body, measured by an accelerometer attached to the injector body. The main drawback of this method is that injector body vibrations due to the injector opening transient often do not decay by the time the injector closes, making it difficult to distinguish between the opening and the closing transients. This method also requires an accelerometer permanently attached to each injector.
The most preferred form of the present invention is based on measuring stress waves that are only generated at the exact moments when the injector valve opens or closes. Therefore, in the most preferred form of the present invention, signals due to these two events do not overlap and the opening and closing times can be determined with high accuracy and with minimal computation. Additionally, the most preferred form of the present invention produces numerically accurate measurements of the intensities of the opening and closing transients of the injector valve and it does so with only one sensor per engine.
The art of stress wave measurement is only known to a relatively small community of practitioners as opposed to measurement of vibrations that is well known and widely used.
The term vibration refers to motion of a body in a fashion where all or a significant portion of the body's mass is moving. In an internal combustion engine, for example, there are significant vibrations at the rotational frequency of the crankshaft and at the engine firing frequency. Excitation of engine vibrations requires significant forces and the vibrational motion involves significant energy.
Vibrations can be measured with accelerometers that are attached to the vibrating body. A piezoelectric accelerometer 5 is shown schematically in FIG. 1. The sensor is enclosed in housing 1. Piezoelectric crystal 2 is attached to the bottom of housing 1. Mass 3 is attached to the top of piezoelectric crystal 2. When housing 1 vibrates in the vertical direction with acceleration a, mass 3 applies force m×a on piezoelectric crystal 2, where m is the size of mass 3 measured in units of mass. The applied force generates strain in piezoelectric crystal 2 and said crystal generates electrical charge in response to the strain. The charge is proportional to force m×a and, therefore, is also proportional to acceleration a. Electrical leads 4 can be used to connect the charge to electronic processing circuitry, not shown in FIG. 1, that converts the charge to voltage proportional to acceleration a.
Unlike vibrations, stress waves are elastic waves contained within the solid that comprises the body. These waves are generated by short-duration impacts of the body and they move at the speed of about 5000 m/s through a metallic body. Stress waves in solids can be generated by impacts that involve very low forces and, consequently, the generated waves involve very low amounts of energy as they move through the impacted body. For example, measurable stress waves can be excited in an engine block just by tapping it lightly with a finger. The theory of stress waves generation and propagation is explained in detail in the book Stress Waves in Solids by Herbert Kolsky, published by Dover Publications in 1963.
Stress waves in solids can be measured with piezoelectric, fiber-optic, MEMS and other stress-wave sensors. FIG. 2 shows schematically one embodiment of a piezoelectric stress-wave sensor 9 formed in accordance with a preferred embodiment of the invention. The sensor is housed in housing 6. The sensing element is piezoelectric crystal 2. Piezoelectric crystal 2 is permanently attached to face plate 7 that is also the bottom of housing 6. The space inside housing 6 is filled with filler 8 to keep piezoelectric crystal 2 in place and to prevent vibration of the internal components of the sensor. When strain 10 is applied to face plate 7, it reaches piezoelectric crystal 2 and piezoelectric crystal 2 generates electrical charge proportional to strain 10. Signal leads 4 are used to connect the generated charge to electronic processing circuitry not shown in FIG. 2. Note that FIG. 2 is only a schematic representation that excludes design details that are required for high gain and low noise measurements of stress waves.
Stress-wave sensor 9 in FIG. 2 incorporates design features that make its response to case acceleration negligible. These features include crystal material selection, shape of the crystal, and the use of filler 8. Consequently, when sensor-wave sensor 9 undergoes motion that involves acceleration, signal leads 4 do not carry a measurable charge signal due to the acceleration.